Transmitters used in optical fiber communications systems typically require a package containing a semiconductor laser coupled to an optical fiber that extends from the package. A major challenge in constructing such transmitters is in maintaining optimal alignment of the laser with the optical fiber such that a maximum part of the laser output can be transmitted through the fiber. A common approach is "active alignment" in which, for example, .the laser is bonded to a substrate, and the optical fiber is incrementally moved until a maximum part of the laser output is directed through the fiber, whereupon the optical fiber is permanently bonded to the same substrate. Alternatively, the fiber can be first bonded to the substrate, with the laser being moved into alignment and then bonded.
For a variety of reasons, solder is the favored material for bonding both the optical fiber carder and the laser carrier to the substrate. During active alignment, the solder contacting the device being aligned is melted or reflowed, the device is held in alignment, and the solder is cooled and solidified. A long recognized problem with this process is that thermal stresses caused by differential thermal expansion of the various elements can be manifested as small movements of the device even after the solder has solidified, resulting in small misalignments. As described, for example, in the U.S. patent of Blonder et al., U.S. Pat. No. 5,307,434, granted Apr. 26, 1994, and in references cited therein, one way of reducing such stresses is to localize the heat applied to reflow the solder by using in the solder element bonding pad a tiny resistive heater for generating the heat.
Especially when one seeks to align a laser with a single mode optical fiber, tiny misalignments may significantly reduce the portion of the laser light coupled to the optical fiber. Even with the improvements described in the Blonder et al. patent, for example, misalignments can still occur due to latent thermal stresses produced during the solder reflow step. We have tried using a plurality of small resistively heated solder elements, rather than only one, in an effort to reduce further the thermal stresses. While this provides some improvement, slight misalignments are still a problem.